IP(3) receptors: toward understanding their activation

Intercellular Ca(2+) waves: mechanisms and function

Intercellular calcium (Ca(2+)) waves (ICWs) represent the propagation of increases in intracellular Ca(2+) through a syncytium of cells and appear to be a fundamental mechanism for coordinating multicellular responses. ICWs occur in a wide diversity of cells and have been extensively studied in vitro. More recent studies focus on ICWs in vivo. ICWs are triggered by a variety of stimuli and involve the release of Ca(2+) from internal stores. The propagation of ICWs predominately involves cell communication with internal messengers moving via gap junctions or extracellular messengers mediating paracrine signaling. ICWs appear to be important in both normal physiology as well as pathophysiological processes in a variety of organs and tissues including brain, liver, retina, cochlea, and vascular tissue. We review here the mechanisms of initiation and propagation of ICWs, the key intra- and extracellular messengers (inositol 1,4,5-trisphosphate and ATP) mediating ICWs, and the proposed physiological functions of ICWs.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.

Inositol 1,4,5-trisphosphate receptors and pacemaker rhythms

Intracellular Ca(2+) plays an important role in the control of the heart rate through the interaction between Ca(2+) release by ryanodine receptors in the sarcoplasmic reticulum (SR) and the extrusion of Ca(2+) by the sodium-calcium exchanger which generates an inward current. A second type of SR Ca(2+) release channel, the inositol 1,4,5-trisphosphate receptor (IP(3)R), can release Ca(2+) from SR stores in many cell types, including cardiac myocytes. However, it is still uncertain whether IP(3)Rs play any functional role in regulating the heart rate. Accumulated evidence shows that IP(3) and IP(3)R are involved in rhythm control in non-cardiac pacemaker tissues and in the embryonic heart. In this review we focus on intracellular Ca(2+) oscillations generated by Ca(2+) release from IP(3)R that initiates membrane depolarization and provides a common mechanism producing spontaneous activity in a range of cells with pacemaker function. Emerging new evidence also suggests that IP(3)/IP(3)Rs play a functional role in normal and diseased hearts and in cardiac rhythm control. Several membrane currents, including a store-operated Ca(2+) current, might be activated by Ca(2+) release from IP(3)Rs. IP(3)/IP(3)R may thus add another dimension to the complex regulation of heart rate.

Termination of Ca²+ release for clustered IP₃R channels

In many cell types, release of calcium ions is controlled by inositol 1,4,5-trisphosphate () receptor channels. Elevations in concentration after intracellular release through receptors (R) can either propagate in the form of waves spreading through the entire cell or produce spatially localized puffs. The appearance of waves and puffs is thought to implicate random initial openings of one or a few channels and subsequent activation of neighboring channels because of an “autocatalytic” feedback. It is much less clear, however, what determines the further time course of release, particularly since the lifetime is very different for waves (several seconds) and puffs (around 100 ms). Here we study the lifetime of signals and their dependence on residual microdomains. Our general idea is that microdomains are dynamical and mediate the effect of other physiological processes. Specifically, we focus on the mechanism by which binding proteins (buffers) alter the lifetime of signals. We use stochastic simulations of channel gating coupled to a coarse-grained description for the concentration. To describe the concentration in a phenomenological way, we here introduce a differential equation, which reflects the buffer characteristics by a few effective parameters. This non-stationary model for microdomains gives deep insight into the dynamical differences between puffs and waves. It provides a novel explanation for the different lifetimes of puffs and waves and suggests that puffs are terminated by inhibition while unbinding is responsible for termination of waves. Thus our analysis hints at an additional role of and shows how cells can make use of the full complexity in R gating behavior to achieve different signals.

Calcium signals are important for a host of cellular processes such as neurotransmitter release, cell contraction and gene expression. While the principles of activation and spreading of calcium signals have been largely understood, it is much less clear how their spatio-temporal appearance is shaped. This issue is of high relevance since the spatio-temporal signature is thought to carry the information content. In our paper we study the dynamical mechanisms that determine the time course of calcium release from receptor channels. We use a stochastic channel description combined with a recently developed model for the distribution of released calcium in a microdomain. The simulations uncover a complex control mechanism, which allows for the tuning of release from short frequent puffs to extended and less frequent wave-like release. Unexpectedly, the model predicts that for wave-like release the dissociation of from the receptors leads to termination of the calcium signal. This effect relies on a well-known gating property of R channels, which earlier has been regarded as superfluous in studies for groups of channels. Our results also provide a missing link to understand cellular response to calcium-binding proteins and present a novel mechanism for information processing by R channels.

Timescales of IP(3)-evoked Ca(2+) spikes emerge from Ca(2+) puffs only at the cellular level

The behavior of biological systems is determined by the properties of their component molecules, but the interactions are usually too complex to understand fully how molecular behavior generates cellular behavior. Ca(2+) signaling by inositol trisphosphate receptors (IP(3)R) offers an opportunity to understand this relationship because the cellular behavior is defined largely by Ca(2+)-mediated interactions between IP(3)R. Ca(2+) released by a cluster of IP(3)R (giving a local Ca(2+) puff) diffuses and ignites the behavior of neighboring clusters (to give repetitive global Ca(2+) spikes). We use total internal reflection fluorescence microscopy of two mammalian cell lines to define the temporal relationships between Ca(2+) puffs (interpuff intervals, IPI) and Ca(2+) spikes (interspike intervals) evoked by flash photolysis of caged IP(3). We find that IPI are much shorter than interspike intervals, that puff activity is stochastic with a recovery time that is much shorter than the refractory period of the cell, and that IPI are not periodic. We conclude that Ca(2+) spikes do not arise from oscillatory dynamics of IP(3)R clusters, but that repetitive Ca(2+) spiking with its longer timescales is an emergent property of the dynamics of the whole cluster array.

New paradigms in cAMP signalling

Signalling through adenosine 3'5' monophosphate (cAMP) is known to be important in virtually every cell. The mapping of the human genome over the past two decades has revealed an unexpected complexity of cAMP signalling, which is shared from insects to mammals. A more recent technical advance is the ability to monitor intracellular cAMP levels at subcellular spatial resolution within the time-domains of fast biochemical reactions. Thus, new light has been shed on old paradigms, some of which turn out to be multiple new ones. The novel aspects of cAMP signalling are highlighted here: (1) agonist induced plasticity - showing how the repertory of cAMP signalling genes supports homeostatic adaptation; (2) sustained cAMP signalling after endocytosis; (3) pre-assembled receptor-Gs-adenylyl cyclase complexes. Finally, a hypothetical model of propagating neuronal cAMP signals travelling form dendrites to the cell body is presented.

Intracellular Ca(2+) channels - a growing community

The Ca(2+) signals that control almost every cellular activity are generated by regulating Ca(2+) transport, usually via Ca(2+)-permeable channels, across the plasma membrane or the membranes of intracellular organelles. The most widespread and best understood of the intracellular Ca(2+) channels are inositol trisphosphate receptors (IP(3)R) and ryanodine receptors, most of which are expressed in the endoplasmic or sarcoplasmic reticulum. However, accumulating evidence suggests physiological roles for many additional Ca(2+) channels in both ER and other intracellular organelles. Interactions between these channels, whether mediated by Ca(2+) itself or interactions between proteins, is a recurrent feature of the Ca(2+) signals evoked by physiological stimuli. We focus on two specific examples, clustering of IP(3)Rs and NAADP (nicotinic acid dinucleotide phosphate)-evoked Ca(2+) release from endo-lysosomes, to illustrate the diversity of Ca(2+) channels and the interplay between them.

Physiological mechanisms of signal termination in biological systems

Studies on the regulation of cellular activity mainly focus on signal generation, but termination of signalling is an equally important factor, which prevents inappropriate activity. This paper reviews the mechanisms, which can cause termination of signalling, and provides examples that illustrate the importance of these processes. Inactivation of voltage-gated Na(+) channels and the photoactivated rhodopsin molecule is caused by rapid conformational rearrangements. Negative feedback can also contribute to the termination of signalling for various mechanisms, including plasma membrane ion channels or cAMP signal generation. In immune cells, the tyrosine-based inhibitory motif (ITIM)-containing molecules are essential negative regulatory components. Desensitization of G-protein-coupled receptors can occur with homologous and heterologous mechanisms, mediated by β-arrestin molecules and second messenger-induced kinases respectively. In NF-κB signalling, resynthetized IκB and other enzymes form negative feedback loops. GTPase-activating proteins are also dedicated to termination of signalling, because they can switch off the small G proteins by increasing their endogenous GTP hydrolysis. In many systems, signal termination is a result of a combined action of several different mechanisms, which underlines the importance of these processes.

Tonic signaling from O₂ sensors sets neural circuit activity and behavioral state

Tonic receptors convey stimulus duration and intensity and are implicated in homeostatic control. However, how tonic homeostatic signals are generated and how they reconfigure neural circuits and modify animal behavior is poorly understood. Here we show that Caenorhabditis elegans O(2)-sensing neurons are tonic receptors that continuously signal ambient [O(2)] to set the animal's behavioral state. Sustained signaling relied on a Ca(2+) relay involving L-type voltage-gated Ca(2+) channels, the ryanodine and the inositol-1,4,5-trisphosphate receptors. Tonic activity evoked continuous neuropeptide release, which helps elicit the enduring behavioral state associated with high [O(2)]. Sustained O(2) receptor signaling was propagated to downstream neural circuits, including the hub interneuron RMG. O(2) receptors evoked similar locomotory states at particular O(2) concentrations, regardless of previous d[O(2)]/dt. However, a phasic component of the URX receptors' response to high d[O(2)]/dt, as well as tonic-to-phasic transformations in downstream interneurons, enabled transient reorientation movements shaped by d[O(2)]/dt. Our results highlight how tonic homeostatic signals can generate both transient and enduring behavioral change.

Structural and functional conservation of key domains in InsP3 and ryanodine receptors

Inositol 1,4,5-trisphosphate receptors (InsP₃R) and ryanodine receptors (RyR) are tetrameric intracellular Ca²⁺ channels¹. For each, the pore is formed by C-terminal transmembrane domains and regulated by signals detected by the large cytosolic structures. InsP₃R gating is initiated by InsP₃ binding to the InsP₃-binding core (IBC, residues 224-604 of InsP₃R1)² and it requires the suppressor domain (SD, residues 1-223)^2-8. We present structures of the N-terminal region (NT) of InsP₃R1 with (3.6 Å) and without (3.0 Å) InsP₃ bound. The arrangement of the three NT domains, the SD, IBC-β and IBC-α, identifies two discrete interfaces (α and β) between the IBC and SD. Similar interfaces occur between equivalent domains (A, B and C) in RyR1⁹. The orientations of the three domains docked into a tetrameric structure of InsP₃R¹⁰ and of the ABC domains in RyR⁹ are remarkably similar. The importance of the α-interface for activation of InsP₃R and RyR is confirmed by mutagenesis and, for RyR, by disease-causing mutations^9,11,12. InsP₃ causes partial closure of the clam-like IBC, disrupting the β-interface and pulling the SD towards the IBC. This reorients an exposed SD loop (HS-loop) that is essential for InsP₃R activation⁷. The loop is conserved in RyR and includes mutations associated with malignant hyperthermia and central core disease^9,11,12. The HS-loop interacts with an adjacent NT, suggesting that activation re-arranges inter-subunit interactions. The A-domain of RyR functionally replaced the SD in a full-length InsP₃R, and an InsP₃R in which its C-terminal transmembrane region was replaced by that from RyR1 was gated by InsP₃ and blocked by ryanodine. Activation mechanisms are conserved between InsP₃R and RyR. Allosteric modulation of two similar domain interfaces within an N-terminal subunit re-orients the first domain (SD or A-domain), allowing it, via interactions of the second domain of an adjacent subunit (IBC-β or B-domain), to gate the pore.

Calcium signaling: from single channels to pathways

Ca(2+) is not only one of the most versatile and ubiquitous second messengers but also a well-established representative example of cell signaling. The identification of most key elements involved in Ca(2+) signaling enables a mechanistic and quantitative understanding of this particular pathway. Cellular behavior relies in general on the orchestration of molecular behavior leading to reliable cellular responses that allow for regulation and adaptation. Ca(2+) signaling uses a hierarchical organization to transform single molecule behavior into cell wide signals. We have recently shown experimentally that this organization carries single channel signatures onto the whole cell level and renders Ca(2+) oscillations stochastic. Here, we briefly review the co-evolution of experimental and theoretical studies in Ca(2+) -signaling and show how dynamic bottom-up modeling can be used to address -biological questions and illuminate biological principles of cell signaling.

Inositol 1,4,5-trisphosphate and its receptors

Activation of cells by many extracellular agonists leads to the production of inositol 1,4,5-trisphosphate (IP₃). IP₃ is a global messenger that easily diffuses in the cytosol. Its receptor (IP₃R) is a Ca(2+)-release channel located on intracellular membranes, especially the endoplasmic reticulum (ER). The IP₃R has an affinity for IP(3) in the low nanomolar range. A prime regulator of the IP₃R is the Ca(2+) ion itself. Cytosolic Ca(2+) is considered as a co-agonist of the IP₃R, as it strongly increases IP(3)R activity at concentrations up to about 300 nM. In contrast, at higher concentrations, cytosolic Ca(2+) inhibits the IP₃R. Also the luminal Ca(2+) sensitizes the IP₃R. In higher organisms three genes encode for an IP₃R and additional diversity exists as a result of alternative splicing mechanisms and the formation of homo- and heterotetramers. The various IP₃R isoforms have a similar structure and a similar function, but due to differences in their affinity for IP₃, their variable sensitivity to regulatory parameters, their differential interaction with associated proteins, and the variation in their subcellular localization, they participate differently in the formation of intracellular Ca(2+) signals and this affects therefore the physiological consequences of these signals.

Diospyrin derivative, an anticancer quinonoid, regulates apoptosis at endoplasmic reticulum as well as mitochondria by modulating cytosolic calcium in human breast carcinoma cells

Diospyrin diethylether (D7), a bisnaphthoquinonoid derivative, exhibited an oxidative stress-dependent apoptosis in several human cancer cells and tumor models. The present study was aimed at evaluation of the increase in cytosolic calcium [Ca(2+)](c) leading to the apoptotic cell death triggered by D7 in MCF7 human breast carcinoma cells. A phosphotidylcholine-specific phospholipase C (PC-PLC) inhibitor, viz. U73122, and an antioxidant, viz. N-acetylcysteine, could significantly prevent the D7-induced rise in [Ca(2+)](c) and PC-PLC activity. Using an endoplasmic reticulum (ER)-Ca(2+) mobilizer (thapsigargin) and an ER-IP3R antagonist (heparin), results revealed ER as a major source of [Ca(2+)](c) which led to the activation of calpain and caspase12, and cleavage of fodrin. These effects including apoptosis were significantly inhibited by the pretreatment of Bapta-AM (a cell permeable Ca(2+)-specific chelator), or calpeptin (a calpain inhibitor). Furthermore, D7-induced [Ca(2+)](c) was found to alter mitochondrial membrane potential and induce cytochrome c release, which was inhibited by either Bapta-AM or ruthenium red (an inhibitor of mitochondrial Ca(2+) uniporter). Thus, these results provided a deeper insight into the D7-induced redox signaling which eventually integrated the calcium-dependent calpain/caspase12 activation and mitochondrial alterations to accentuate the induction of apoptotic cell death.

Analysis of IP3 receptors in and out of cells

BACKGROUND

Inositol 1,4,5-trisphosphate receptors (IP3R) are expressed in almost all animal cells. Three mammalian genes encode closely related IP3R subunits, which assemble into homo- or hetero-tetramers to form intracellular Ca2+ channels.

SCOPE OF THE REVIEW

In this brief review, we first consider a variety of complementary methods that allow the links between IP3 binding and channel gating to be defined. How does IP3 binding to the IP3-binding core in each IP3R subunit cause opening of a cation-selective pore formed by residues towards the C-terminal? We then describe methods that allow IP3, Ca2+ signals and IP3R mobility to be examined in intact cells. A final section briefly considers genetic analyses of IP3R signalling.

MAJOR CONCLUSIONS

All IP3R are regulated by both IP3 and Ca2+. This allows them to initiate and regeneratively propagate intracellular Ca2+ signals. The elementary Ca2+ release events evoked by IP3 in intact cells are mediated by very small numbers of active IP3R and the Ca2+-mediated interactions between them. The spatial organization of these Ca2+ signals and their stochastic dependence on so few IP3Rs highlight the need for methods that allow the spatial organization of IP3R signalling to be addressed with single-molecule resolution.

GENERAL SIGNIFICANCE

A variety of complementary methods provide insight into the structural basis of IP3R activation and the contributions of IP3-evoked Ca2+ signals to cellular physiology. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Spatial organization of intracellular Ca2+ signals

The ability of Ca(2+), the simplest of all intracellular messengers, selectively to regulate so many cellular behaviours is due largely to the complex spatiotemporal organization of intracellular Ca(2+) signals. Most signalling pathways, including those that culminate in Ca(2+) signals, comprise sequences of protein-protein interactions linked by diffusible messengers. Using specific examples to illustrate key principles, we consider the roles of both components in defining the spatial organization of Ca(2+) signals. We discuss evidence that regulation of most Ca(2+) channels by Ca(2+) contributes to controlling the duration of Ca(2+) signals, to signal integration and, via Ca(2+)-induced Ca(2+) release, to defining the spatial spread of Ca(2+) signals. We distinguish two types of protein-protein interaction: scaffolds that allow rapid local transfer of diffusible messengers between signalling proteins, and interactions that directly transfer information between signalling proteins. Store-operated Ca(2+) entry provides a ubiquitous example of the latter, and it serves also to illustrate how Ca(2+) signals can be organized at different levels of spatial organization - from interactions between proteins to interactions between organelles.

Gene expression in accumbens GABA neurons from inbred rats with different drug-taking behavior

Inbred Lewis and Fisher 344 rat strains differ greatly in drug self-administration; Lewis rats operantly self-administer drugs of abuse including nicotine, whereas Fisher self-administer poorly. As shown herein, operant food self-administration is similar. On the basis of their pivotal role in drug reward, we hypothesized that differences in basal gene expression in GABAergic neurons projecting from nucleus accumbens (NAcc) to ventral pallidum (VP) play a role in vulnerability to drug-taking behavior. The transcriptomes of NAcc shell-VP GABAergic neurons from these two strains were analyzed in adolescents, using a multidisciplinary approach that combined stereotaxic ionotophoretic brain microinjections, laser-capture microdissection (LCM) and microarray measurement of transcripts. Laser-capture microdissection enriched the gene transcripts detected in gamma-aminobutyric acid (GABA) neurons compared to the residual NAcc tissue: a ratio of neuron/residual >1 and false discovery rate (FDR) <5% yielded 6623 transcripts, whereas a ratio of >3 yielded 3514. Strain-dependent differences in gene expression within GABA neurons were identified; 322 vs. 60 transcripts showed 1.5-fold vs. 2-fold differences in expression (FDR < 5%). Classification by gene ontology showed that these 322 transcripts were widely distributed, without categorical enrichment. This is most consistent with a global change in GABA neuron function. Literature mining by Chilibot found 38 genes related to synaptic plasticity, signaling and gene transcription, all of which determine drug abuse; 33 genes have no known association with addiction or nicotine. In Lewis rats, upregulation of Mint-1, Cask, CamkII , Ncam1, Vsnl1, Hpcal1 and Car8 indicates that these transcripts likely contribute to altered signaling and synaptic function in NAcc GABA projection neurons to VP.

Ca2+ signaling during mammalian fertilization: requirements, players, and adaptations

Changes in the intracellular concentration of calcium ([Ca(2+)](i)) represent a vital signaling mechanism enabling communication among cells and between cells and the environment. The initiation of embryo development depends on a [Ca(2+)](i) increase(s) in the egg, which is generally induced during fertilization. The [Ca(2+)](i) increase signals egg activation, which is the first stage in embryo development, and that consist of biochemical and structural changes that transform eggs into zygotes. The spatiotemporal patterns of [Ca(2+)](i) at fertilization show variability, most likely reflecting adaptations to fertilizing conditions and to the duration of embryonic cell cycles. In mammals, the focus of this review, the fertilization [Ca(2+)](i) signal displays unique properties in that it is initiated after gamete fusion by release of a sperm-derived factor and by periodic and extended [Ca(2+)](i) responses. Here, we will discuss the events of egg activation regulated by increases in [Ca(2+)](i), the possible downstream targets that effect these egg activation events, and the property and identity of molecules both in sperm and eggs that underpin the initiation and persistence of the [Ca(2+)](i) responses in these species.